How do I save a blender file as a MP4 in 2.80 video editing?
The following configuration produces an .mp4 file with codecs that are recommended by the YouTube guidelines and their recommended upload encoding: 


Output format: FFmpeg video
Container: MPEG-4
Video codec: H.264
Audio codec: AAC


Adjust the output path to where you want the video saved. Then press CTRL+F12 or use Render > Render Animation in the UI to save the video.